1. Preheat oven to 375ºF, and lightly grease muffin pan with butter or oil.
2. Saute chopped onions in a pan with the oil until onions are translucent.
3. Crack eggs into a bowl and whisk. Fill about 1/3 of muffin cups with spinach, and sprinkle about a teaspoon of the sauteed onions on top.
4. Pour whipped eggs into muffin cups and fill them about 2/3 full, then add a pinch of grated cheddar cheese.
5. Bake for about 15 minutes, until golden on top.
6. Gently remove them from pan with butter knife, then serve.